Getting started: Risk and Reward

Hello teacher

I have read some books and heard from traders
You must define a reward and risk system in order to be successful in trading.
And they say you must know your average gains and your average losses in order to determine the stop loss
Through the average of your past earnings and determine the appropriate reward and risk scheme for you, as well as the time period for the gains and the time period for the loss.
I had a headache in this matter. If I knew my average earnings, I wouldn’t read the book. If I knew my average gains, I wouldn’t have recorded in your training. If I were winning from the market, I would have known my average earnings.
Is this issue as important as they claim or do I get past this topic because I had a headache for reward and risk.